This application is related to U.S. Patent Applications entitled xe2x80x9cHigh Purity Fluid Delivery System,xe2x80x9d xe2x80x9cFlowmeter for the Precision Measurement of an Ultra-Pure Material Flow,xe2x80x9d xe2x80x9cMethods of Manufacturing a PFA Coriolis Flowmeter,xe2x80x9d xe2x80x9cManufacturing Mass Flow Meters Having a Flow Tube Made of a Fluoropolymer Substancexe2x80x9d and xe2x80x9cCompensation Method for a PFA Coriolis Flowmeter,xe2x80x9d each filed on the same day as this application. The entire disclosures of the referenced applications are incorporated by reference herein.
1. Field of the Invention
The present invention relates generally to fluid flow measurement and control, and more particularly, to Coriolis mass flow controllers that are suitable for use in ultra-pure or corrosive applications, or other applications not compatible with standard metal Coriolis flowmeters.
2. Description of Related Art
Many industries such as semiconductor, pharmaceutical, and bio-technology experience fluid delivery problems due to the typically low flow rates, the use of abrasive chemical fluids, the use of corrosive chemical fluids, and the need for contaminant free, accurate, compact, and real-time fluid delivery and/or blending systems.
A fluid delivery system generally consists of three components: fluid propulsion, flow measurement and control, and a user interface. Many present systems use a positive displacement pump, such as a peristaltic pump, to perform all three tasks. The pump propels the fluid from the storage container to the process or reactor. The pump also moves the fluid at a more or less constant rate depending on the speed of the pump, though the peristaltic pumping action causes a pulsation in the fluid delivery rate. The user interface consists of adjusting the pump""s speed or simply turning the pump on and off. This method does not provide very precise flow control and the pumping action and the internal geometry of the pump can contaminate or harm the fluid.
The peristaltic pump provides no closed loop feedback on the flow measurement. In addition, since it is a volumetric delivery system, the amount of fluid varies with changing process conditions such as pressure, temperature, etc. The pump tubing also wears over time, changing the volume of fluid delivered with no change in pump speed. When the process requires precise fluid delivery it is also common to verify the delivery rate by manually measuring the amount of the fluid on a scale or graduated container over a period of time. A typical batch blending system is shown in FIG. 1. Multiple fluids, A through N, flow into a container 11 placed on a scale 12. One fluid is allowed to run through a flow valve 13 at a time. The scale total is examined and when the desired amount of Fluid A has been added, the valve 13 is closed. The same process is repeated with the remaining fluids. Eventually, a total mixture is obtained. If too much or too little of any fluid has been added the process must continue until the proper mass of each fluid, within some acceptable error band, has been added.
Another known approach uses a level sensor to measure the volume of each fluid of the blend as it is being added to the vessel. This requires a very precise knowledge of the volume of the vessel with small increments of vessel height.
Chemical-Mechanical Planarization (CMP) is a critical process in the semiconductor industry that involves a process to flatten the wafer surface of a semiconductor by applying an ultra-pure fluid containing suspended solid particles and a reactive agent between the wafer surface and a polishing pad. In most applications, the polishing pad rotates at a controlled speed against the semiconductor to flatten the surface. Over-polishing the wafer can result in altering or removing critical wafer structures. Conversely, under-polishing of the wafer can result in unacceptable wafers. The polishing rate of the wafer is highly dependent upon the delivery rate of the fluid and the total amount of fluid delivered during a polishing operation.
Another process used in the semiconductor industry requiring accurate control of fluid flows and a contaminant free environment is the photolithography process. As is known in the art, photolithography is a process that applies a light sensitive polymer, known as resist, or photo resist, to the wafer surface. A photomask containing a pattern of the structures to be fabricated on the wafer surface is placed between the resist covered wafer and a light source. The light reacts with the resist by either weakening or strengthening the resist polymer. After the resist is exposed to light the wafer is developed with the application of fluid chemicals that remove the weakened resist. Accurate and repeatable resist delivery is essential to properly transfer the pattern. The resist must be contamination free as any xe2x80x9cdirtxe2x80x9d on the surface will cause a defect in the final pattern.
A modification of this process applies a host of new liquids to the wafer surface to create films that will become an integral part of the final semiconductor. The primary function of these films is to act as an insulator between electrical conducting wires. A variety of xe2x80x9cspin-onxe2x80x9d materials are being evaluated with a wide variety of chemical compositions and physical properties. The key difference between the lithography process and the spin-on deposition is that any defect in the film (such as a void, bubble or particle) is now permanently embedded in the structure of the semiconductor and could result in non-functioning devices and a financial loss for the semiconductor producer.
Both of these processes take place in a tool called a xe2x80x9ctrack.xe2x80x9d The purpose of the track is to apply a precise volume of fluid to the surface of a stationary or slowly spinning wafer. Additional chemical processing steps may be used to convert the liquid to the proper structure. After the liquid application, the wafer rotation speed is rapidly increased and the liquid on the wafer surface is spun off the edge. A very thin, consistent thickness of liquid remains from the center of the wafer to the edge. Some of the variables that affect liquid thickness include the resist or dielectric viscosity, solvent concentration in the resist or dielectric, the amount of resist/dielectric dispensed, speed of dispense, etc.
The track will also provide additional processing steps after liquid application that changes the liquid to a polymer using a bake process that also removes any solvent in the film. The track also controls the environment around the wafer to prevent changes in humidity or temperature and chemical contaminants from affecting the performance of the film. Track system performance is determined by the accuracy and repeatability of liquid delivered to the wafer surface in addition to minimizing defects in the film caused by voids, bubbles and particles.
Therefore, there is a need for an efficient, compact and contaminant free solution to fluid delivery systems to address shortcomings associated with the prior art.
In one aspect of the present invention, a mass flow measurement and control device includes an enclosure with a Coriolis mass flowmeter situated therein. The Coriolis mass flowmeter has a flow-tube made of a high-purity plastic material, a driver coupled to the flow tube for vibrating the flow tube, and a pickoff coupled to the flow tube for sensing Coriolis deflections of the vibrating flow tube. A pinch valve includes an elastomeric tube made of a high-purity plastic material in fluid communication with the flow tube. An actuator with a rain operatively connected thereto is situated adjacent the elastomeric tube, and a reference surface is positioned generally opposite the ram such that the elastomeric tube is squeezable between the ram and the reference surface.
The flow tube and pinch valve elastomeric tube may both be fashioned from PFA. Further, these tubes may comprise a single tube. In other embodiments, the pinch valve tube is fashioned from a more flexible material, such as silicone. In some exemplary embodiments, a controller that receives a setpoint signal and an output signal from the Coriolis flowmeter and provides a control output signal to the pinch valve actuator in response thereto. The controller may be situated in the enclosure, or external thereto. Similarly, the pinch valve may be positioned within the enclosure, or attached to an external surface thereof